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Would hydrogen fuel tanks on an airplane increase lift?
Since hydrogen is lighter than air, would hydrogen fuel tanks help provide lift for the airplane?

The amount of lift generated would be tiny for all the trouble and expense involved.   Look how much hydrogen was required to lift the Hindenberg!

@abelenky: Actually the lift would not change. Only the weight could.

Let's do some quick math. Air has a density of about 1.3 g/L and hydrogen has a density of about 0.09 g/L. The best case for lifting force would be storing the hydrogen at the same pressure as the surrounding air. This means that every L of hydrogen can displace 1.21 g of air, reducing the density of the aircraft. The amount of air displaced by the aircraft (and therefore the buoyancy) would not change, but the density of the aircraft in that space would decrease. This would decrease the lift needed, but let's see by how much.
The greatest amount of free volume in an aircraft (that isn't part of the pressurized section of the fuselage) is the fuel tanks. An A380 has a fuel capacity of about 323,000 L. If we consider filling half of that with hydrogen, this would result in a total reduction of about 190 kg. An A380 has a max takeoff weight of 575,000 kg. This change would help but would be very small compared to the total lift needed.
As David Richerby points out, hydrogen needed as fuel would need to be pressurized (if not liquified) to fit enough onboard. This would increase the density above that of any displaced air, which would instead add weight to the aircraft.
Perhaps the hydrogen could be produced instead of stored. Newer aircraft do have something called an OBIGGS, or onboard inert gas generating system. This system generates nitrogen which is used to fill the space in fuel tanks after the fuel is used. Being an inert gas, nitrogen can lower the amount of oxygen in the space below what would be required for ignition of the fuel vapors.
However, this system splits nitrogen out of air, considering that our atmosphere is mostly nitrogen. Hydrogen would have to come from somewhere else, usually water, which would add more weight. Another problem with hydrogen is that when added to air it can be explosive all by itself, which would make the situation even worse. Considering the minor benefit, this would not be a system worth installing on an airliner.
Regarding the feasibility of a hydrogen powered aircraft, NASA did a study and decided that this is not a good source of power for an aircraft. The volume and weight required to store even liquid hydrogen would be too much for the aircraft to be very useful.

The buoyancy is going to be even less because if you wanted to use that hydrogen as fuel, you'd have to pressurize it to get enough on board.

Also, I'm not sure what the relevance of OBIGGS is. You couldn't inert your hydrogen buoyancy tank because the inerting gas would significantly increase its mass. And you certainly couldn't inert it if it was intended to be used as fuel!

I'm just making the point that sometimes aircraft do get filled with gas, but you are right that hydrogen makes no sense in that context.

As long as the aircraft shape does not change, the neither does the amount of displaced air!

Hydrogen has a higher specific energy density than JetA, so if you could ignore the weight of the needed high pressure cryogenic tanks hydrogen will weigh less per megajoule. However even liquefied H2 is much less dense than JetA so there is less energy per unit volume.

Buoyancy (a.k.a. static lift) depends on the volume of the object. If you put something else in the tanks, the buoyancy does not change.
What can change, however, is the weight. If you wanted to use it as fuel, you'd have to take such amount as to get the same combustion heat as you get from kerosene. Hydrogen has lower¹ heating value 119.96 MJ/kg and kerosene has ~43 MJ/kg (²), so you would need almost three (2.79) times less hydrogen by weight. This is totally unrelated to its density, but it is the only way how it can offer advantage as fuel.
However:

You'd still need to squeeze a lot of it into the aircraft, which means either huge pressure or very cold.
Even liquid hydrogen has density of only 70.85 kg/m³ (gaseous at 300 bar has only 26.96 kg/m³) compared to 800 kg/m³ for kerosene, so even for three times less weight you'd still need four times larger fuel tanks and they would come with higher form drag and higher weight. They would also have slightly higher buoyancy, but the drag penalty would be higher than any gain from that.
Dealing with stuff colder than −253 °C is complicated and it takes a lot of energy to keep it cool it and keep it cold. For spacecrafts the weight advantage is so important that its worth it, but for aircraft it isn't.
Compressed hydrogen is not really an option. The tanks would be heavier than what you could save in the weight of fuel.

So overall hydrogen does not have any advantage to offer for aircraft. It only has it for spacecraft that leave the atmosphere quickly so they don't care about aerodynamic drag much, but care about weight a lot.

¹ Note that “lower heating value” is a technical term. Fuels have two heating values, lower, which is for leaving the products in gaseous state, and higher, which includes the enthalpy of condensation if the products would be liquid at room temperature and standard pressure, as is the case with common combustion product, water.
² Unlike hydrogen, kerosene is a mixture with some tolerance for composition, so it does not have a more accurate heating value, because it is allowed to vary from batch to batch.

See also here.

Your first paragraph is correct, assuming that the volume of the aircraft doesn't change. However, simply making the aircraft much larger and using uncompressed (or only lightly compressed) Hydrogen gas is an option that actually would actually increase buoyancy. Of course, it's not feasible for airliners due to the massive drag increase and the risk of the whole thing going up in a big fireball (which, at least after the first few seconds, yields a dramatic penalty to lift.)

@Bianfable, you are right. Thanks.

No. If you want to use hydrogen as fuel, you need a lot of it, which means you need to pressurize it. Hydrogen is less dense than air when it's at ordinary atmospheric pressure but, as you start to pressurize it (i.e., fit a greater mass of hydrogen into a fixed volume), that advantage decreases. Worse, if your fuel is pressurized, you need to store it in a strong tank and strong means heavy.

Taking the numbers from @fooot's answer: Air is 13.3 times denser that hydrogen. As consequence, hydrogen in a tank at at a pressure of already 13.3 bar has the same density as the ambient air. That common gas bottles everybody knows are initially filled with 200-300 bar. So, that advantage decreases really fast.

It would increase lift if it were unpressurized. Of course, it would also increase volume by a few orders of magnitude if it were going to produce enough lift to matter in the context of a significantly-sized aircraft, which would lead to form drag penalties that far outweighed the lift benefit for craft that fly anything close to the speeds used in modern airliners. This can, of course, work for much slower aircraft like airships, though such use can have undesirable consequences.
If it's pressurized, it will be more dense than air and, therefore, will not create lift.
The aforementioned undesirable consequences look something like this:

A picture is worth a thousand words.

Only three words in this case.

Bringing emotional arguments into this are we

@jjack Not sure I follow you. Clearly, the aircraft in that incident was storing hydrogen. This is the primary reason that, after that point, airships started using helium instead of hydrogen, even though helium is much more difficult to acquire. The "We don't like to die in a large fireball" argument generally carries over quite well to modern aircraft and is a driving consideration for several modern aircraft design decisions, including the lack of storing large amounts of hydrogen on board.

@jjack Because of the problem that caused the Hindenburg disaster, modern airships are filled with helium, not hydrogen, even though helium is much more difficult to acquire. Still not quite sure I understand your argument. What do you mean by the argument (hydrogen burns very easily in the presence of oxygen which is plentiful in our atmosphere) does "not really carry over to storing hydrogen on aircraft?"

@jjack Please provide a (non-conspiracy-theory) citation for your claim that sufficient quantities of hydrogen to use as a fuel in an airliner can be stored safely in an airliner in a practical manner. Particularly when it's stored in such a way that it additionally provides the lift asked about in this question.

@jjack Saying "You're fake news" is not a citation. And it's also not acceptable under Stack Exchange's Be Nice policy. It's fine to post a comment if you think something is wrong in an answer, but you need to back that up with something other than your opinion and you need to do it politely.

@jjack Sorry, I wasn't trying to imply that you're known for spreading conspiracy theories. I just meant that I was looking for a source for your claim that isn't some conspiracy-theory site (i.e. a reputable source.) I mentioned that only because 99.9% of the "we should be using something other than kerosene" 'sources' I see are some "we could all be using this miracle fuel, but 'big oil' suppresses it" conspiracy theory nonsense and about 0% of them are from actual aerospace engineers.

@jjack Also, as someone who is an engineer and is rather familiar with aviation, physics, chemistry, and the application of physics and chemistry to aviation, I'm rather skeptical of your claim that there's a safe and practical way to store uncompressed or lightly compressed hydrogen in an aircraft that flies around at Mach 0.85 or so and moves on the surface at 150-200 mph.

That fireball was mainly the skin material.

@MaxPower Regardless of the source of the visible fire (over which there seems to be some debate,) the most commonly-accepted theory (and the one most consistent with the observations) of the initial source of the fire as well as the reason for its rapid spread was the hydrogen fuel.


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How do aircraft altimeters calculate altitudes accurately while airborne?
While an aircraft is airborne, the local barometric pressure can change, causing an altimeter to no longer display the correct altitude. How are altimeters re-calibrated to display the correct altitude, or there are other methods to accomplish that?

i dont think altimeters displays the correct values  Why not?  What do you mean?

in general, as I have heard, due to the changing air around the aircraft, altimeters settings go inaccurate and thus they need to be re-tuned.

Not sure what you mean... Once an aircraft descends below the transition level, they receive local weather information which includes air pressure. The pilot adjusts the altimeter to local air pressure and thus has a locally correct altitude shown. See also ATIS on how they usually receive such data.

I have asked a similar question here: http://aviation.stackexchange.com/questions/13087/how-does-an-altimeter-deal-with-the-non-linear-pressure-gradient

Altimeters are calibrated to a standard atmosphere model (International Standard Atmosphere, ISA).
What the altimeter shows you is the vertical distance between the altitude equivalent to your current pressure (pressure altitude) and a reference "pressure altitude". The reference pressure altitude is set in pressure units (hPa or In. Hg) on the altimeter subscale. This is known as the "altimeter setting", set in the "altimeter setting window".

Image credit: FAA Pilots Handbook of Aeronautical Knowledge
Flying at lower altitudes (below "transition altitude"), the altimeter is set to airport/regional QNH, which is a pressure calculated so that if you were standing on the measurement point, the altimeter would indicate your exact elevation. At other altitudes, it would approximate altitude above mean sea level, although it would show incorrectly if temperature differed from ISA.
Above transition altitude, the altimeter is set to QNE, or standard pressure (1013.25 hPa, or 29.92 In. Hg). The altimeter will now only indicate correctly if both temperature and pressure are in accordance with ISA. You are now flying on "flight level" (where the FL is your QNE indication in hundreds of feet, ie 20,000 ft = FL200).
For the purpose of avoiding obstacles, both QNH and QNE have to be corrected using various methods, which are described in various manuals. (QNH for temperature, QNE for both temperature and pressure, both for other possible factors such as wind etc). For instance, this diagram shows three different aircraft, all flying the same pressure altitude, but with three different true altitudes:

Image credit: FAA Pilots Handbook of Aeronautical Knowledge
However, altitude is also used for the purpose of traffic separation, and for this purpose, neither QNH or QNE is corrected. This might sound counter-intuitive, but as long as your altimeter is showing the same error as any conflicting traffic, you can be safely separated, therefore: The altitude shown on the altimeter is not corrected at all (except by calibration for possible aerodynamic interference, such as compressibility, venturi-effects etc).
In technically sophisticated aircraft, barometric altitude input is used for vertical navigation. Since vertical navigation is concerned with the true altitude of the aircraft, the aircrafts Air Data Computer (ADC) will calculated a true altitude based on information available to it (either through sensors, or through pilot input). The true altitude is however, as stated, not shown on the pilots primary flight instruments.

Good info. Additionally, GPS can indicate true altitude, and terrain-following radar can produce very accurate AGL measurements below transition. Both are used in the latest commercial airliners to avoid CFIT.

I think the simplest way to view the use of non-corrected flight levels is to view them as representing certain pressures, and observe that regions of uniform pressure corresponding to different flight levels will always have a substantial distance between them in at least one direction.  If planes at different flight levels have 0 feet of vertical separation but 10 miles of horizontal separation, the lack of vertical separation won't be a problem.

There are two separate answers to this question, depending on what altitude the flight is conducted.
At lower altitudes, below the transition altitude (18,000' in the US, but it varies from country to country), it is the responsibility of the pilot to receive the local "altimeter setting" -- which is the local barometric pressure, corrected to sea level.  So if you are preparing to takeoff from an airport on the high pressure side of the front, that might be 30.30 inches of mercury, and this value would be set in the altimeter, and at this point the altimeter should display the field elevation for where you are.
Now let's say that you fly across the front into an area of much lower barometric pressure -- perhaps 29.80" Hg.  That is now what is set into the altimeter, and upon landing your altimeter will display the altitude of the airport where you've landed.
Note that if you did this all at once, going from 30.30 to 29.80, you'd be dialing in a very large change into your altimeter, which would change the indicated altitude by about 500 feet, meaning that you were that far off of the altitude you thought you were flying, and that's a bad thing.  So what happens in reality is that every few dozen miles, you'd get an updated altimeter setting from any of the various weather reporting facilities available -- ATC, automated ASOS broadcasts, data-linked weather, FSS, etc.  So you'd adjust from 30.30 to 30.25, then later to 30.31, then later to whatever the next station was reporting, and so on -- each a much smaller adjustment.
For aircraft flying at high altitudes (and high groundspeeds), this process becomes impractical, so a different solution is used.  Passing through the transition altitude, a standard setting of 29.92" Hg is entered into the altimeter, and so now everyone is flying at a constant altitude above NOT mean sea level, but above a reference plane.  And altitude is no longer reported at 15,000', but as Flight Levels -- FL 190 for what would otherwise be called 19,000', etc.
This means that FL FL400 isn't really 40,000' above sea level most days, and on some days it's higher than that, and other days it's lower, but as long as everybody assigned to fly at FL400 is at the same altitude (which they are, all having 29.92 set in their altimeters), everything works.  (Particularly since everybody at FL400 is 1000' above everybody at FL390 -- that's what we really care about!)

AIUI -- transition alt in the US is 10,000', not 18,000'

@UnrecognizedFallingObject - nope, 18,000 transition altitude, FL180 transition level in the USA.  At least, that's where we reset the altimeters earlier today...

Transition altitude in Australia is 10,000. In the US it is indeed 18,000 and in Europe it varies but can be as low as 3000. Why the difference? It's determined by the highest elevation terrain in the respective areas (Alaska is ignored for that purpose in the USA.) The transition altitude is chosen to be above the highest fixed object you can hit, so above the transition altitude you are only concerned about other aircraft, not clearance above terrain.

@RalphJ -- interesting...my memory must be off for some reason

Altimeters are connected to a static pressure port, a little vent that's outside the aircraft. This senses the pressure in the outside air. For the altimeter to update with the correct altitude, it senses differences and changes in air pressure. As an aircraft climbs, the pressure decreases, and the altimeter registers an increase in altitude.
But what happens if you're on the ground, and the atmospheric pressure changes over time? You need to calibrate your altimeter.
Since altimeters are extremely sensitive to air pressure, a slight change can register a big difference. There is normally a knob, which lets you calibrate it. It allows you to adjust the readings in the air pressure, known as Inches of Mercury. Generally, the expectation is that this setting is at 29.92 inches of mercury.
As aircraft move into areas with differences in air pressure, pilots have to adjust the altimeter to account for this. Pilots will normally be notified by the ground when flying at local altitudes, for example during approach or 10 000 AGL. Re-adjusting is simple: just turn the knob.

As seen in the picture, you have the altimeter. To the bottom-left is the knob to adjust the readings. The little numbers on the right indicate the current observation in Inches of Mercury. Here, it is 29.92.

but we do have digital altimeters as well, so they also needs to be re-calibrated? also, what is the exact precision do these altimeters goes ?

Digital altimeters also have to be re-calibrated, because they still use a static pressure port. When calibrated correctly, these altimeters are normally very precise. See this question for that: http://aviation.stackexchange.com/questions/3727/how-precise-are-altimeters

I think that the question has been answered by a couple of great answers, so there's only one thing I can add.
In my opinion the main reason for the transition from QNH to QNE is so that above the transition altitude, the airplane, engine and systems always perform the same way.
For example, for every possible load-out, there's a certain optimal altitude for the plane to fly at. In truth, this altitude is really a pressure, that only converts into an altitude by assuming a standard pressure above a simulated reference plain.
In other words, it doesn't matter how hot or cool it is or where around the globe you are, if you convert your outside pressure into a reference altitude, you can always fall back to the same performance tables.

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Is there any work on improving fuel tanks so they could store hydrogen?
I think that hydrogen is among the best fuels for use in supersonic and hypersonic flight, is there any work on improving fuel tanks so they could store hydrogen?
What would be the best state to store hydrogen for use as an aircraft fuel: solid, liquid or gas?
What are the complexities for storing it at each state?

In gaseous form, the amount of -- the mass of -- hydrogen you could store in the volume of aircraft fuel tanks would be negligible. In order to store enough mass of hydrogen, you'd have to store it liquified, which is how it's stored in rockets.
In order for that to be useful, though, you'd have to have engines entirely redesigned in order to burn cryogenic fuels, which would be a massive undertaking.  I'm not aware of any serious interest in using cryogenic fuels in aircraft today.  Kerosene works well and, compared to liquid hydrogen, is cheap and relatively safe to transport and store.
Rockets use cryogenic fuels because they absolutely need all of the "bang for the buck" that they can possibly get, so the high cost and extra safety measures are accepted in that realm as a necessary cost of doing business. This thread discusses rockets using cryogenic fuels or a mixture of cryogenic fuels and kerosene.  The bottom line, they often use cryogenic fuels -- especially in upper stages -- in order to get more payload to orbit.
Unless something changes in the economics of kerosene vs cryogenic fuels, however, it seems improbable that aircraft will switch to liquid hydrogen on any great scale.

Comments are not for extended discussion; this conversation has been moved to chat.

First, to debunk a myth regarding the Hindenberg, which always gets brought up whenever hydrogen is mentioned. Hydrogen is less safe than liquid fuel, but it is not less safe than natural gas or propane. Hydrogen has a higher flame propagation velocity than hydrocarbon gasses, but contains much less energy per unit volume than hydrocarbon gasses. A huge airship filled with natural gas would be just as hazardous as a huge airship filled with hydrogen, yet we have natural gas pipework running to our houses and with proper controls there are very few incidents.
Nevertheless, hydrogen is not a good fuel for aircraft for the following reasons.
(Source: Wikipedia)
Above shows that hydrogen contains about 142MJ/kg of energy compared with about 42MJ/kg for kerosene. However it contains a lot less energy per unit volume. Liquid hydrogen contains only 10 MJ/litre while kerosene contains 33MJ/litre. Therefore hydrogen fuel tanks would need to be more than 3 times the size of kerosene fuel tanks. Gaseous hydrogen at atmospheric pressure would occupy hundreds of times more space, while hydrogen at 700bar is only about a factor of 2 worse than liquid hydrogen, but the weight of a vessel for holding 700bar pressure would negate the weight advantage of hydrogen.
Therefore the most practical way of storing hydrogen in an aircraft would be in the liquid state, as is used in spacecraft such as the Space Shuttle. Use of hydrogen would result in some weight savings and hence fuel savings. However the fuel tanks would be so big that the savings would be partially negated by the the additional drag (not to mention the additional weight of the larger tanks.)
Hydrogen is expensive to produce and liquefy. Most hydrogen is produced by reforming natural gas and steam into hydrogen by the reaction CH4 + 2H2O = CO2 + 4H2, which is a highly energy intensive process. Therefore, although a hydrogen fuelled plane might consume less energy than a kerosene fuelled plane, this does not take into account the energy used in the process of producing and liquefying the hydrogen, nor the cost involved in doing so. Overall, hydrogen fuel is likely to be more expensive.
Hydrogen is also more difficult to handle. hydrogen tanks are never perfectly insulated, so hydrogen boils off, and the tanks ice up. It is also, more hazardous than liquid fuel if a spill occurs, as it will evaporate and create an explosive mixture in air (though, as I have said above, this issue also applies to natural gas.) Liquid hydrogen is much colder than liquefied natural gas, and so creates more problems with materials, boiloff and icing.
Industrial gas turbines can run on hydrogen without too much modification. But with the extensive certification procedures for aviation, modifying an aircraft engine for hydrogen will be quite costly. Also the faster flame velocity of hydrogen means that NOx is more difficult to control and this is likely to cause problems with emissions regulations.
In conclusion, the only way hydrogen is ever going to be viable in the aviation industry in general is if it turns out to be easier to use in a fuel cell driven craft than a hydrocarbon fuel.
If economically feasible, a fuel cell / electric motor drive would be more efficient than a combustion jet engine. That said, the practical and safety issues mean that the aircraft industry will be one of the latest adopters of such technology.

Some proposed using electrolysis of water to convert surplus electricity (from the grid) into hydrogen. In this case, no need for natural gas.

@RodrigodeAzevedo in principle yes, but in the current situation better to use the electricity to charge electric cars where battery weight is not an issue. Using electricity in cars is much easier than in planes. Hydrogen from  electrolysis is more expensive than hydrogen from methane, but if and when fossil fuels are phased out, hydrogen from electrolysis using renewably generated electricity would make sense.

The weight of a vessel for holding 700bar pressure would negate the weight advantage of hydrogen.: How much weight does a vessel add? Is it three, ten or rather thirty times the weight of the contained hydrogen?

@Gypaets I hadn't worked it out but: according to Wikipedia there is an aluminium alloy called 2014-T6 with a yield strength of 414MPa, stronger than most steel. Assuming a safety factor of 1.5, a 1m radius cylindrical tank would need to be 70MPa/(414MPa/1.5)= 0.253m thick. Density of liquid hydrogen 70kg/m3, 700bar gaseous hydrogen is about half this according to chart above. Hydrogen mass per metre length= (PI11)35kg/m3=35kg. Aluminium mass per metre length (2PI*0.253)*280kg/m3 = 445kg. So about 12.5 times. Obviously it depends a lot on the material, maybe carbon fibre will do better.

"yet we have natural gas pipework running to our houses and with proper controls there are very few incidents." This isn't really a very good argument. Home rarely fly around at large fractions of Mach 1, nor do they frequently get into crash landings. Using a fuel that combusts more easily can dramatically lower survival rates. Consider runway excursion accidents and accidents like Asiana 214, for example. Accidents that were very survivable would have been much less survivable if the fuel had burned. Pretty much the only way to prevent that is to have a fuel (like Jet-A) that's hard to burn.

Building pipes that don't start fires (or explode) while remaining perfectly stationary is much easier than building tanks that can contain high-pressure fuels during high-energy crashes without starting a fire or exploding.

@reirab I think I have covered that in my answer. Hydrogen is less safe than liquid fuel, but it is not less safe than natural gas or propane.  and It is also more hazardous than liquid fuel if a spill occurs, as it will evaporate and create an explosive mixture in air (though, as I have said above, this issue also applies to natural gas.) I would not recommend using hydrogen, nor liquefied natural gas as an aviation fuel for the reasons you state. But the straw man argument regarding the Hindenberg has inevitably appeared on this page and I felt the need to address it.

You're saying that an alternative power source that seems, on the surface, to be more efficient, economical, and "environmentally friendly" at the point of use has hidden inefficiencies, costs and environmental impacts that aren't readily obvious to the casual observer? I find that hard to believe!! </sarcasm>

Consider the nature of liquid hydrogen. In a liquid, non boiling state, it must be at a temperature of 22 Kelvin, which is -253C/-423F. Extremely cold. This calls for heavily insulated containers, and an accidental release could be extremely dangerous. A few gallons of liquid hydrogen spilled on you would turn you into an ice cube almost instantly.
It is true that when hydrogen is burned with oxygen, the result is water (plus a bit of ozone). However, when hydrogen is burned with atmospheric air, which is more nitrogen than oxygen, then some nasty oxides of nitrogen are also produced, so it is a 'clean' fuel only when it is provided with pure oxygen, or used in a fuel cell.
Today, liquid H is used as fuel in a few specialized situations that benefit from its high energy density: largely rockets, although it has also been used (combined with fuel cells) in some AIP submarines. These are areas where the additional complexity and expense are outweighed by the benefit.
In commercial use, it would be handled every day in high volume, so the complexities of dealing with this extremely cold substance would be magnified, as would the dangers of accidental release. And there is still the issue of the pollutants left over from burning hydrogen in the atmosphere.
In his book Skunk Works, Ben Rich describes an effort to retool the A-12 Oxcart aircraft (later to become the SR-71) to run on liquid hydrogen, to extend its range. He reported that by the time they had added the heavy insulated tanks, the range of the A-12 would have been extended only a couple of percent, plus the complexities of midair refueling, so Kelly Johnson killed the project and sent the project money back to the government. That was with 1960's technology, better methods may be available today, but that does illustrate how hydrogen as an aircraft fuel isn't quite the wonder solution it might seem to be.
On top of that, consider that free hydrogen doesn't exist on the earth in any great volume. It must be manufactured by electrolyzing water, which takes a lot of electricity, and that adds to the cost. Hydrogen can also be produced by catalyzing natural gas, although done in volume, there would be a lot of leftover chemicals to dispose of.
Should a practical fusion reactor be developed, then it is possible that hydrogen as a fuel for general transportation use would become viable. In aircraft, it could be utilized in fuel cells to power electric motors, to bypass the pollution issue.

Nitrogen oxides is just by product of high pressure, temperature and lean combustion, and is not a direct consequence of certain kind of fuel. 2) Hydrogen is manufactured mostly chemically, e.g. hydrocarbons or carbon plus water. And what chemical leftovers? CO2? Hydrogen has been made in large scale for a hundred years already. 3) Fusion? Half a century into nuclear energy now only a few dozen ships in the world runs nuclear power, let alone controlled fusion itself is also half a century away. And how does that have anything to do with fuel cell?

A hydrogen fuel tank, by nature, must be cylindrical.  To save weight it must be short and stout, not long and narrow.
On an aircraft, it must also be at the center of gravity, so it does not throw the aircraft out of balance as fuel is spent.  This also necessitates a short tank, not a long length-of-fuselage tank (all the hydrogen would roll fore or aft during maneuvering, crashing the aircraft).
Hydrogen is a very low density fuel, so the fuel tanks are enormous.  The lion's share of the tank space used on the Shuttle main tank was for the hydrogen.
That means there is only one possible place for a hydrogen fuel tank on an aircraft: the center of the fuselage (typically over the wings).  Pretty much the entire width of the fuselage.  What's left for the passengers to sit in?  The very front and rear fuselage.  That means separate crews too - passing through the tank area would be impracticable, unless the aircraft had a "blister" up top like a 747.
This also defeats all the design efficiencies of carrying the fuel in the wings.  That means the wings need to be stronger to carry the lift to where the fuel weight is. That matters.

It would seem that the more sensible approach would be two tanks at both ends of the fuselage.

@MSalters  Hmm, but that puts the fuel mass at fore and aft extremes, which makes CG a huge issue that you have to manage very well, with good defense-in-depth and crews trained to manage it well. If bungled it crashes the plane. It also puts the fuel in the worst possible location structurally, having to carry structural support the farthest. At the wing root is bad enough - fuel really is best in the wings. (countless small tanks?)

Hydrogen is pretty lightweight; the main issue with the tanks is their volume. You need big spherical tanks to reduce losses (square-cube law); putting many small tanks in the wings means that the surface area of the tanks is much bigger. As for wing lift, a lifting body approach might make sense.

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Can hydrogen gas be an alternative fuel source for a gas turbine engine?
I want to do research for an alternative fuel source for a commercial aircraft that will reduce the cost for flights.
Will the hydrogen gas be ignited in the combustion chamber and produce enough thrust to power the aircraft for flying? As for the problem that people had stated before for storing the gasses in a high pressure gas tank, can an increase in the aircraft weight be reduced by replacing the tank with usage of a hydrogen generator instead of a whole bulky tank that will weigh more?
By using a generator which converts water into hydrogen gas, we can just use water as our main fuel source which is more cost efficient?
In a flame of pure hydrogen gas, burning in air, the hydrogen (H2) reacts with oxygen (O2) to form water (H2O) and releases energy. If carried out in atmospheric air instead of pure oxygen, as is usually the case, hydrogen combustion may yield small amounts of nitrogen oxides, along with the water vapor.

Your energy to split the H2O still has to come from somewhere. Thus, you still have to store something that will provide you with that energy. Then there's a question of if converting that stored energy into a form that's useful for you to split H2O is the most efficient use of that energy, or can it be converted into thrust in another, more efficient, manner, rather than using it to generate hydrogen which you are going to burn. In general, the process you've described has significantly lower efficiency than other possibilities.

@Makyen +1 This is also known as No Free Lunch or the Second Principle of Thermodynamics.

"By using a generator which converts water into hydrogen gas": Same as Is water a possible fuel for jet engines?. The other question has got a score of -4, yours +5, that's funny. Burning one kg of fuel creates 42 MJ. To do the same with water you need 3.5 kg of water, but also more than 42 MJ of energy to extract the 350 g of hydrogen they contain and burn it (source). This is not a good deal (neither for mass nor energy saving).

If you "want to do a research for an alternative fuel source for a commercial aircraft that will reduce the cost for flights", I strongly suggest that you take one or two university-level courses in Thermodynamics.  No one has ever beaten the Second Law of Thermodynamics.

If you have a magic generator that converts water to hydrogen for free, why not go all the way and use one that takes water and CO2 (from the atmosphere) and coverts them conveniently to jet fuel?

@mins: In all fairness, that one has even more fundamental mistakes (sea water?!) while this one has at least a sane title.

@Flydog57: "No one has ever beaten the Second Law of Thermodynamics" doesn't mean we don't have some seemingly counter experiments we can't yet explain.

I always thought of the three laws like this: You can't win.  You can't break even. You can't get out of the game!

@CrossRoads, you're listing the "Laws of Poker in Hell". (p 27 of the book linked in the comment preceding yours). Alternatively some say "There are no perfect refrigerators" or "Situations tend to progress from bad to worse" (ibid.). As the author notes, there are not less than 21 (serious) formulations of the second law of thermodynamics more or less compatible, and counting...

@mins fraud, experimental errors, external input of energy missed by the experimenters. those 3 explain every single "perpetual motion" and "free energy" mechanism out there.

@jwenting: I wouldn't be so affirmative. I know layman perpetual motion stories and the likes are a priori ridiculous. but physics is still a very partially understood domain (say 5% or 10% is currently satisfactorily theorized), to the point scientists had to invent a "dark matter" which is from many points of view as surprising as layman perpetual motion (85% of the universe made of something unknown as of today, we can't interact with, but is required to explain galaxy red shift and Newton's free fall).

@mins having a degree in physics, I am more confident in the validity of the laws of thermodynamics than you maybe. As to dark matter, that's effectively a placeholder for things not yet understood and is well understood to be just that.

@jwenting: If you remember, Einstein was a fervent opponent (not to say more) to Copenhagen ideas like wave-particle duality or uncertainty, and he was wrong, and Bohr and Heisenberg were right.

@mins those were new, poorly understood, concepts at the time and there was much debate about what was actually going on. The laws of thermodynamics are rather better established...

Yes, and it has been demonstrated 30 years ago on the Tupolev 155. This is/was a hydrogen-powered version of the Russian Tu-154B tri-jet. Only one has been built and has since been retired after demonstrating the use of liquid hydrogen in 5 experimental flights. In total, the Tu-155 performed about 100 flights with several fuels, among them hydrogen and liquefied natural gas.

Tu-155 (top), cut-out (center) and layout (bottom). Source: https://www.aviaru.net/pr/?id=11633
Hydrogen is actually better for a jet engine: It is gaseous at normal temperature, so there is no delay for the evaporation step to happen as there is for liquid fuels before mixing and combustion can start. Also, hydrogen burns in a wide variety of mixing ratios with oxygen, so flame-outs are much less likely, making a smaller combustion chamber possible.
The downsides, of course, are storage and the small molecular size of hydrogen. It is very hard to contain, and it needs big volumes to store a given amount of chemical energy. Pressurized storage at 200 bars holds only 18 kg/m³ or 45 times less than kerosene. With 142 MJ/kg, hydrogen holds three times as much chemical energy than kerosene, but then the volumetric efficiency of kerosene is still a factor of 13.3 better.
Cryogenic storage swaps pressure for low temperature: Below 33 K and above 13 bar, hydrogen becomes a liquid and storage density increases to 30 kg/m³. Still, cryogenic hydrogen storage needs 4 times the volume of the same amount of energy stored as kerosene, plus the isolation and the energy to cool it down and compress it.

They used the Tu154 because its center of gravity is very aft, due  to the engines being aft.  They need to place the hydrogen tank at the center of gravity (so fuel burnup doesn't affect trim). With the Tu154 CG being so far aft, it allowed  a single cabin space instead of two half-cabins fore and aft of the tank, which would be a big mess.

Evaporation of kerosene is a complete non issue at the temperature of the air exiting the compressor. The issue with liquid fuels is atomisation. That is, getting a fine spray of small droplets. When fuel nozzles become dirty with deposits, the spray pattern can deteriorate.

@Harper: Right, and a smaller type like the Tu-134 would had less volumetric efficiency, so the larger one offered the better choice. An Il-62 would had worked, too.

@Penguin: Thanks for finding the better word; I was less concerned about the heating itself than the time delay it causes. Droplet size is already small when fuel is injected at high pressure trough lots of fine nozzles, but until all of the droplet is evaporated, the mixture travels some distance downstream inside the combustion chamber. And the gaseous fuel is atomized as well.

Re "cryogenic hydrogen storage needs 8 times the volume of the same amount of energy stored as kerosene": How did you come to 8 times? The energy density of jet fuel is 3.7 times that of liquid hydrogen (37 MJ/l vs. 10 MJ/l)

The cut-away picture is very small (no complaints, probably the only one available), but it appears that the majority of the cabin is taken up with machinery and storage, leaving very little space for cargo - self-loading or otherwise. Therefore, while powering the plane may be more efficient, flying it from place to place becomes pointless, except for gaining hours for the pilots. That's good, but not going to be of much interest to commercial carriers of any sort.

@FreeMan: The 155 was a research aircraft and the production version would have been the Tu-156. It was full of instrumentation and never intended for revenue service. Only the big blue tank would remain in the 156. Regarding the pictures: You are right, they are awfully small, but I found no bigger version which was accepted by Imgur.

@PeterMortensen: Thanks for pointing that out! I should double-check my sources.

@PeterKämpf - points well taken. However, that big blue tank still takes up a lot of revenue space while leaving the space inside the wings essentially useless.

@FreeMan: Couldn't agree more. I still think the future of civil aviation is in synthetic fuels made by hydrogenating carbon dioxide using solar energy. Like plants do it. If only the deserts of this world would be more politically stable …

@PeterKämpf eh... for all the noise being made lately, the American Southwest isn't that bad, and from what I hear, there's a large chunk of the middle of Australia that's not good for much beyond driving across to say you did. :)

@PeterKämpf do you know what happened to Tu-156? was it stop only due to the fall of soviet union or they actually can't proceed with the production because of other reason or limitation?

@cat: I have no inside information. I think the main reason is really the fall of the Soviet Union which had all Russian (and Ukrainian) aircraft design bureaus enter a dark period. However, while some designs were good enough to weather that storm, the Tu-156 showed no commercial promise and could not secure any private funding. In a way, that would be the other limitation.

@cat it's simply not economical. Range goes down, cabin space goes down, and you now have to have major storage facilities for the cryogenic fuels at airports, and elsewhere. And because you need to transport it to the airport in cryogenic form, you'd better make it there, meaning a manufacturing plant at every airport, using electricity, air, and natural gas as inputs and producing hydrogen gas and carbon dioxide (yes, that's how you make hydrogen gas in bulk, electrolysis of water is way too inefficient).

13 bar? Do we really need that pressure? Why not go 1-2bar and <20K

@Abdullah Sure we can. But then we need to cool down the hydrogen even further. This becomes harder the closer we get to zero K. Also, isolation becomes more important. In the end, it is easier to go to 13 bar and require less cooling.

@PeterKämpf there's really gonna be a benefit to that anymore?

Hydrogen works just fine on rockets. However "just fine" on rockets doesn't mean it is practical on an aircraft.
The only way you can utilize $\mathrm{H}_2$ is storing it cryogenically. This is because $\mathrm{H}_2$ goes supercritical at $-240\,{}^\circ\mathrm{C}$, and no matter how hard you squeeze it beyond this temperature it would refuse to liquify and thus remain a low number density fluid. Of course, you could store it as a supercritical fluid, but that would require an incredibly heavy pressure vessel.
If you agree that you must store $\mathrm{H}_2$ cryogenically, then take a look at this. What do you think are your odds of carrying all this hardware onboard an airplane and still have usable payload?
Your trouble doesn't end there. To burn $\mathrm{H}_2$ as a fuel, you must move it out of the tank and into the engine. And to do that, you must use some sort of pump, and a pump must have some moving parts that are immersed in the liquid that it's supposed to pump. And here comes the trouble: you are pumping a liquid that boils at $-240\,{}^\circ\mathrm{C}$, and even the tiniest surface imperfection, the tiniest burr, the tiniest machining mark, the tiniest grooves and troughs on the surface of the immersed moving pump creates minuscule surface vortices, and these vortices heats up the liquid $\mathrm{H}_2$ near the part's surface so that it boils, forming bubbles, which merge, split and collapse thousands of times per second, and the minuscule pressure pulses sent out by these events impacts on the already extremely cold thus brittle moving parts of your pump, chipping it almost instantly, and after chipping the damaged pump will stir the entire stream of liquid $\mathrm{H}_2$ to a violent boil and blow itself off.
Liquid $\mathrm{H}_2$ is the most difficult fuel to handle, and even rockets steer away from it whenever possible. It is one thing to use it on something that only lasts for a few hundreds of seconds, quite another on something that lasts tens of thousands of hours.
EDIT: I almost forgot. Hydrogen, its molecule being so small, diffuses like crazy, even within the "solid" objects to the naked eyes like steel, titanium, copper, and aluminum. So all metal parts are inevitably impregnated with hydrogen with use and form hydrides with it, causing it to decrease in strength. So good luck with the whole fuel system! The entire aircraft will be a literal ticking time bomb.

If LH2 is such a problem to pump around, why not let it boil and then travel to the engines as a gas?

@StephenS that's an interesting idea because the phase transition is accompanied by a rise in pressure and you wouldn't need a pump. Nevertheless it would still be very cold with all the problems that entails.

@meatball-princess there's another option. You can use electrostatic forces from other atoms to bind the hydrogen in chains. There's a class of these that works quite well: hydrocarbons. But enough of me being a smartass - thanks for the boil off info, that's fascinating!

@StephenS bad idea – gaseous hydrogen diffuses extremely quickly in metals. the only way you can get hydrogen under control is when it's a cold liquid. that's the only way. and remember even then it's got almost no viscosity, a needle hole in a barrel, and the entire barrel of LH2 will be gone in less than a minute!

FYI hydrogen cars use gaseous hydrogen

@Antzi yes they do. They are also just cars, with a small demand on power, torque and hence fuel & air mass flow rate. They can afford to be heavy and inefficient. The hydrogen BMW 7 series' cryogenic hydrogen fuel will be gone in 2 weeks! "Drives like a car, chews through wallet like a space shuttle.", That would have been my slogan for it. All hydrogen cars' hydrogen lines need special attention and frequent inspection.

@MeatballPrincess A car is refueled once every week or two; an airliner is refueled before every flight, so it only needs to keep the fuel contained for a few hours.

@MeatballPrincess That's just not true about frequent inspection and diffusion losses. Long-term high pressure gaseous hydrogen storage works just fine. Current fuel cell cars on the market don't require frequent inspection of the lines.

I think the odds of planes being able to carry "all that hardware" are close to 100%.  The rest of the plane is hardware, you know, the engines are hardware, the kerosene fuel systems are hardware, the flight control systems are hardware, why do you think planes can't carry lots of hardware?

Also I don't think you know what "literal" means.

@rsjaffe yes hydrogen fuel cells are fine. but we are talking about an airplane that needs ~3lbs of hydrogen per second.

@immibis i'm pretty sure a literal ticking timebomb is a literal ticking timebomb. a literal ticking timebomb is a device that, after a period that is more often than not unbeknownst to the poor men that are happened to be at its vicinity, blows up, killing everyone and everything.

The last bit appears to be a triviality. So, you can't use some metals? Use composites, then. Needing a lot of hydrogen per second? Good. Leakage only grows with the radius of a pipe, while flow rate grows with the square of the radius.

@MeatballPrincess Does it make a ticking sound at regularly spaced intervals?

@MSalters LH2 leakage is the worst nightmare, especially in an enclosed space e.g. the mechanical bay of an airplane. H2 in the air is flammable from 4% to 75%, explosive from 18% to 60%, hydrogen flame is dim in the visible light spectrum but extremely hot, hydrogen is odorless, hydrogen is the lightest gas so it has a crazy diffusion speed that forms perfect explosive mixture with ambient air in no time, LH2 has almost zero viscosity so even the tiniest crack cannot hide from it, LH2 is below 240 degrees Celcius so chances are your composite is frozen to a biscuit-like substance.

@MSalters hydrogen flame is completely smokeless and sootless so no use for smoke detectors, LH2 has very low density so LH2 tanks are very bulky, and more tank area=more chance of leakage, LH2 tanks must allow LH2 to constantly boil off, unlike rocket which only accelerates longitudinaly airplanes roll, so LH2 sloshing is something never encountered and solved before. A deeply cryogenic fuel is the last thing you want to live through. A small mistake, then you have a Challenger of a plane.

Using water as the carrier and splitting it on-board to hydrogen and oxygen is a nonstarter. Electrolysis takes vast amounts of electric power, so instead of just water you need to carry water plus (big, heavy) batteries. If you use batteries, you're better off just using electric motors to drive the turbines, that would be lighter than an electrolysis setup.
Hydrogen storage has the same problem as batteries:
Batteries store far less energy per kg, hydrogen stores far less energy per m3 than jet fuel.
Hydrogen also requires heavy, high-pressure tanks.
A better alternative would be converting hydrogen and carbon monoxide into liquid fuel on the ground (using the Fischer-Tropsch process) and burning that in your engines.

Re: "Hydrogen storage has the same problem as batteries: both store far less energy per kg than jet fuel. No, that is not true. Per kg hydrogen has 3.3 times more energy (142 MJ/kg for hydrogen and 43 MJ/kg for jet fuel). (Per volume is a different story, 10 MJ/l and 37 MJ/l, respectively.)

Yes, I meant per volume for hydrogen.

@Hobbes as you stated "you're better off just using electric motors to drive the turbines." Is there actually a motor powerful enough to produce the torque and speed needed to turn the heavy turbine? not trying to sound mean or sarcastic, i'm just really curious about your statement and genuinely want to know if such motor exist. thanks for your answer btw

Electric motors produce up to 10 kW/kg, so 3.5 tons to produce 35 MW (which is in the range of what you need for a large airliner). You'll have to add a fan to that, cowling etc. but (and this surprises me) 10 kW/kg is as good as a modern turbofan.

Hydrogen works just fine as turbine fuel, and does so in space launch turbopumps. Achieving full efficiency, power, and engine life on hydrogen will require tweaks to a pre-existing engine, of course.
On the environmental side, H2 normally burns hotter than hydrocarbons, which produces more N2O, but combustion temperatures can be regulated, and have to be to match turbine life specs.
Storage is the problem. All generators based on storing hydrogen in room-temperature liquids/solids share the problem of considerably worse net:tare ratios than liquid hydrogen tanks. LH2 adds cost and maintenance with cryogenics and still falls short of hydrocarbon energy density.
You can't get net energy by extracting H2 out of water, as that consumes all of the same energy that H2 produces in combustion, plus the losses. Energy cannot be created, only converted. For heat to be created to drive the engine, energy has to be lost elsewhere. In this case it's lost in joining two chemicals, which store potential energy (combustibility) in their state of separation.
If you had a different energy source (actually energy converter) on board, e.g. nuclear, you could use its output to propel the aircraft without the need for combustion.

Re "still falls short of hydrocarbon energy density": in volume yes, but not in mass (though technically specific energy). Hydrogen has 142 MJ/kg whereas jet fuel has 43 MJ/kg (yes, 3.3 times more). Per volume it is 10 MJ/l and 37 MJ/l, respectively.

@PeterMortensen True, I was addressing volumetric density. As for gravimetric, you'd have to compare entire systems complete with storage, insulation, and extra fuselage volume to get a practical estimate of the effects.

Your plan is to not store the hydrogen, but generate the hydrogen inflight.
The problem is that generating hydrogen inflight is even more awkward than storing it.  The component chemicals you combine to make hydrogen are much, much heavier than the hydrogen itself (hardly a surprise since hydrogen is by far the lightest atom)... and both heavier and more expensive than getting the same energy out of petroleum fuel.
What's more, back on the ground, you would need to spend energy to create the component chemicals (to "charge them up") would require electricity or heat, which would require burning other fuel, due to efficiency far more fuel again than the airplane is able to harness.
If the public policy motivation is to reduce fuel spent by airplanes, the greenest way to do that is effective high-speed-rail systems.  Downtown London to downtown Paris is so stacked in favor of rail that I can't believe there are any airflights at all. High speed rail powers directly off electric, efficiently making the most out of green electric sources such as wind or solar.

Alternately, find ways to cleanly synthesize hydrocarbons instead of digging them up.

We can create jetfuel from hydrogen and carbon dioxide, in the Fisher-Tropsch process. However, its very inefficient. Until a vast majority of the world's electricity is produced by renewable or nuclear, it is unlikely that artifically produced hydrocarbons will be used for jet fuel

@CSM: Carbon monoxide, actually. But I agree with the premise. Biofuels are much easier to manufacture, and plants are pretty good at turning carbon dioxide into hydrocarbons.

By using a generator which converts water into hydrogen gas, we can just use water as our main fuel source which is more cost efficient?

NO!
Splitting water into hydrogen and oxygen, done at 100% efficiency, takes exactly as much energy as you get back from burning that hydrogen in the oxygen, at 100% efficiency.  Where are you getting all that energy from to split water?  Your plane would be lighter if you just used that energy source directly to propel the plane and didn't waste time converting water to hydrogen and oxygen and back to water.
In reality, you don't have 100% efficiency, so the water-to-hydrogen-and-oxygen-to-water equipment isn't just dead weight: it's weight that's actively using up some of your power.

It may turn out that compressed hydrogen will be excellent for fixed structures where storage volume is not as great an issue.  Transportation such as trucks or aircraft greater favor higher energy density, or liquid fuels.  Exotic applications in rocketry favor the higher specific impulse per gram of hydrogen compared with hydrocarbon (more bonds to oxidize per unit weight).
Amazingly, conversion of water to hydrogen may make it easier to transport water over long distances as a convertible gas rather than energy intensive pumping and/or building canals.  A 5 psi hydrogen pipeline will cross any mountain range, and, at its destination, provide both fuel for heating/electricity and one gallon of water for every pound of hydrogen burned.
However, issues of volume and extremely low boiling point may limit its applications for large scale transportation.  Liquid natural gas may be a better bet.

"Liquid natural gas may be a better bet." - Jet engine cores are already run on natural gas with no significant modifications, for gas pipeline pumping. And they happily run continuously and maintenance-free for months at a stretch, in locations like Alaska which are inaccessible in winter.

Yes, but would such pipelines have the capacity for the required amount of (equivalent) water? (What would the water be for? Tap water? Irrigation? Pepsi Cola?)

My application is the American Southwest.  Water would be for irrigation and for  household use.

Converting water to hydrogen seems a very Rube Goldberg-esque method. Conversion efficiency is on the order of 50%, pumping losses would have to be huge for the conversion to be more efficient.

@Hobbes yes, but it is better than leaving a windmill idle when the power grid demand is low.  It is a form of energy storage, enabling power companies to design for peak load (more windmills) rather than average.  The H2 goes right onto the pipeline.  In the Southwest, any water is precious.

Ah yes, of course. You'd still have to deal with hydrogen embrittlement of the pipes, though.

We'll have to get to work on liners, I guess.  I'll check in with the nat gas folks and see what they're up to.

I guess the first Aircraft flying with a Turbine, the Heinkel-178, with the Turbine of Hans Joachim Pabst von Ohain, patent US2256198, inventor stated M Hahn, had the jet engine tested with Hydrogen as fuel, the issue with Hydrogen may be storage. www.SAE.org has documents about Hydrogen as Automotive Fuel, no need being member to purchase.

Google "cryoplane final report"  or just cryoplane for a full answer.  Jet liners using cryogenic hydrogen are feasible but won't be cheap unless hydrogen becomes very cheap.  It might do that as renewable energy sources like wind and solar could generate hydrogen by electrolysis when their electricity output is greater than demand the time.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Components on engines, are they removed or blanked out?
As a follow up to why doesn't the A380 use outboard thrust reversers, if a component is not used in an engine, is it completely removed (hardware, wiring etc etc) or is it just made inoperative?
In the case of the A380 wouldn't it make it more difficult to maintain and handle 2 engines which are the same, but with different accessories installed?
Wouldn't it be risky as there might be human error factors whilst servicing or installing the thrust reversers?

Short answer
The reverser is not installed on the outer engines. As the linked question mentions: This allows to lighten the engines, this also prevents any unwanted and hazardous opening in flight as well as the costly check time for an unused feature.
The reverser used on the A380 is a cold flow type. It is merely part of the engine by itself. It is actually integrated in the nacelle structure channeling the bypass cold flow.
An engine should be usable in all four nacelles, the nacelles themselves cannot be swapped as they provide the reverser. The answer to this question seems to confirm this possibility.

Principle
Thrust reversal on this aircraft type consists in opening the bypass duct sideways, blocking the bypass flow in the duct and redirecting it through the opening. The opening is created by sliding the aft section of the nacelle case to expose a set of vanes. The blocked flow now escapes sideways and is guided forward by the vanes:

Principle of cold flow thrust reverser (source)

Thrust reverser on the A380 (source)
Engine, reverser, nacelle
The A380 is equipped with Trent 900 from Rolls-Royce or GP7200 from Engine Alliance. The thrust reversers are made by a different provider, Airbus-Aircelle (part of Safran), which also manufactures the nacelles. This is because the thrust reverser is really integrated in the nacelle rather than linked to the engine.
Trent 900
The reverser is in the cold / bypass flow behind the fan section which is the engine section with the largest diameter (in the Trent 900, about 3m!).

Hot flow, cold flow and reversed flow in a Trend XWB (similar to 900).
Based on RR picture
The cold flow duct (fan duct) is similar in both versions of the nacelles.  The duct is fenced by two semi-annular walls behind the fan frame, where the flow is rushed by the fan. This is called a double D-duct (in reference to the shape of the semi-duct).
The two D-ducts of an inner nacelle contain the reverser panes and their associated components. This version is named the thrust reverser unit (TRU). The other version without the reverser is the fixed fan duct (FFD), it doesn't slide.
TRU

The TRU is visible behind the fan section, source
On the image above, the two halves of the TRU are open. Note the outer wall of the duct has some thickness, contrary to the FFD wall (see next). This volume is used to host the mechanism needed to move the panels rerouting the flow.
The inner wall prevents all the flow to reach the core and the components and pipes around it, in order to not create damage or drag and heat. However a limited quantity of air is still directed toward the core for cooling.
More easy to see on this CG view:

Trent 900 duct and reverser. Based on TurboSquid models.
Top: 3D model by BlueGreen. Bottom: Another excellent 3D model by BlueGreen.
FFD
Left D-duct highlighted in blue.

Nacelle duct for cold flow. Based on image by Benny Zheng on Airliners.net

As I understand it, a large percentage of the thrust produced by this type of engine is from the cold air flow, but I'm not clear. In thrust reversal is the engine still producing forward thrust?

@HowardMiller: You're correct for this type of engine with cold flow reverser. The net rearward thrust is about 40%. This ratio decreases with RPM. See this thread on PPRuNE. This article states an efficiency in the range 30-60%. Efficiency of buckets reverser is higher.

Just out of curiosity, why is the engine so low to the ground in your picture and how did it get there?

@dalearn: It is probably mounted on the older RR testbed aircraft, a 747-200. Unfortunately I don't know the initial source for the image. The engine is brought using a trolley I guess.

Whether an unused component is removed or just left in place depends on how much work it is to remove it, and what the benefit would be.
In the case of the A380, a large advantage of not having thrust reversers on the outboard engines is the reduction in weight and cost, including assembly and maintenance time. This outweighs any added cost of having different parts on those engines.
Since the nacelle without thrust reversers will be lighter, it's likely that the attachment scheme is also different, preventing the wrong one from being installed.
The inboard and outboard engine pylons will already have enough inherent differences that leaving out rest of the systems required to hook up the thrust reversers is a net benefit, and there just wouldn't be anywhere to install them on the wrong engine position.


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To what extent were/are area rule considered when designing the flap-track fairing of transport aircraft?
As you'd know, certain older transport aircraft in the 20th century, such as Convair 990 or Tu-134 featured very distinctive, large anti-shock bodies situated on the trailing edge of the main wing so that the cross-sectional area transition/distribution of the aircraft would be more subtle and thereby reducing wave drag on the high-subsonic/transonic flight regimes.
What got me wondering though, is when designing a flap track fairing on an aircraft that doesn't feature any dedicated anti-shock bodies, how much of it is the result of consideration regarding the area rule/wave drag, as opposed to considering other drags like profile drag?
I know that the flap track fairings as found on most airliners are not designed to act as an anti-shock body, but thought that it would be valid to assume that it will at least have some effects in terms of the area rule and if so, there might be some consideration put into the design regarding it.
It seems like the more recent design trends like in the case of A380, A350XWB and B777X are such that they are trying to make the flap track fairings as small, slim and streamlined as possible and thereby minimize their aerodynamic effects and instead feature other aerostructures, such as an extended wingbox fairing that trails longer behind the actual wingbox to compensate for decreasing cross-sectional area. Are these observations correct?
If it is correct, how does the question in the title apply for older airliners/transports which didn't feature such designs like an extended wingbox fairing? Would it be safe to assume that flap track fairings had a bigger role in achieving smoother transition of cross-sectional area distribution for those older aircraft?

To which extent were/are area rule considered when designing the flap-track fairing of transport aircraft?

Fully, at least from the late Fifties on. While first-generation jets like the Caravelle, Tu-104, DC-8 or the Boeing 707 tried to hide their flap tracks as much as possible, with the VC-10 they became common and indeed started life as anti-shock bodies. The Tupolev designs used their main gear bay as a Küchemann body from the Tu-104 on but added flap track fairings only with the Tu-154.

I know that the flap track fairings as found on most airliners are not designed to act as an anti-shock body

You must be mistaken. They started life as anti-shock / Küchemann bodies and were found to be useful as flap track fairings, too.
One picture should say more than 1000 words. The one below (source) is from VC-10 development and shows the lower side of the wing in a windtunnel experiment without (a) and with flap track fairings (b) at Mach 0.9 with oil added to show the streamlines.

The bright line in (a) is caused by massive separation. Adding the fairings could avoid this separation. Since the added area of the bodies adds friction drag, they are clearly helping to reduce transsonic drag but would increase subsonic profile drag.
You are quite correct: The extended wing box fairing also helps to smooth the cross section distribution, but is also designed to cover landing gear bays and accessories, thereby helping to allow to have a cargo hold with constant cross section over its full length in the lower fuselage. The extended wing box is needed with wing planforms which have a nearly straight trailing edge at their root. Earlier wing designs were trapezoidal, so that all edges were swept over the full wing span and less cross section tailoring was required.
However, as the picture below (source) shows, the Vickers engineers didn't make full use of those large fairings ...

According to the source, those in the picture are anti-shock bodies and not flap track fairings. The layout of the flaps would be also quite "packed" for them to be fairings.

@sophit That is why I added the second picture: Yes, they faired the flap tracks, but modern designs use continuous flaps, not the ones with gaps at each track. Their use as anti-shock bodies is a lucky dual-use. The real anti-shock body is the one between vertcal and horizontal tail.

Thanks for the answer, though may I ask if the above photo of VC-10 wing model under experiment representative of the upper-side of the wingfoil or the lower? I've seen that photo quite a few times and know it's a viscostic flow/boundary layer simulation using an oil, so automatically I'd assume that they are indeed trying to investigate high-subsonic/transonic speeds, but the picture on the right and the boundary layer separation seems like a compression-shock induced separation to me, and to my knowledge, compression shock mainly occurs on the upper-side of the wingfoil.

@MK.s First, it is a windtunnel experiment, not a simulation. Second, the photo shows the lower side of the wing. It was taken at -1° AoA, so a shock on the lower side without Küchemann bodies is quite normal.

Just to be fair with the oldies : Caravelle, DC-8, 707,... had very simple flaps which didn't need fairings in the first place.

@PeterKämpf Thanks for the clarifications! Some clarifications, by "simulations" I meant 'simulating the operating conditions', eg. with a scaled model fo the wing of VC-10, not as in that I've thought it's a computer simulation or sorts. Anyways, great straight forward answers to my questions.


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How valuable is CFI hours outside USA?
CFI serves as an excellent pathway to the airline industry. However, if I don't have the necessary paperwork to remain in the US and work, how does accumulating 1500 hours in single-engine aircraft contribute to advancing into airlines beyond the USA?
I often come across LinkedIn posts from Asian airlines, many of which specify a requirement of 200 or more turboprop hours.
Having obtained a FAA frozen ATPL, I find myself uncertain about the next course of action.

I feel this is better off asked in a discussion forum rather than this specific question and answer page. My 2 cents though: hours are hours and you should be aiming for any job you can get. That might not be an airline job right away. Start by looking in the country you already have the right to work in. Where I'm from in Australia the typical airline job ad requires 500 total time but 250 multi-command hours. If you only have 1500 hours instructing in a Cessna 172 you won't get the job (exceptions apply). Some Asian and European airlines will take you at 200 hours though. It varies so much.


text:
When, exactly, were the DC-8’s airbrakes removed?
The DC-8 was originally equipped with airbrakes on the lower rear fuselage, but these were quickly removed.  Wikipedia says that they were removed before the DC-8 entered service:

[...]  Douglas made a massive effort to close the gap with Boeing, using no fewer than ten aircraft for flight testing to achieve FAA certification for the first of the many DC-8 variants in August 1959.  Much needed to be done: the original air brakes on the lower rear fuselage were found ineffective and were deleted as engine thrust reversers had become available...

Emphasis added.
However, according to the CAB report on the crash of United Airlines Flight 859, the first few DC-8-10s, at least, did have airbrakes when they entered service, and at least one was using them as late as July 1961:

When this action failed to straighten the landing roll he knew the aircraft was going to leave the runway.  He said he used the emergency airbrakes to slow the airplane as much as possible before leaving the runway.  [...]
[...]  The second officer said that after touchdown a series of events occurred which he could not place in sequence with any degree of certainty.  He saw ejector indicator lights blinking, heard a foot thumper, saw the captain reach for the emergency airbrake handle, and saw that the aircraft was heading about 30 degrees off the runway.  [...]  [page 4 of the report]

Emphasis added.
And it’s definitely not misusing the term “airbrakes” to refer to the aircraft’s wing spoilers, since it refers to those as “spoilers” elsewhere in the report.
When were the DC-8’s fuselage airbrakes removed?

The DC-8 is equipped with emergency air brake system. This isn't the type of air brakes you might expect on an airplane, but rather on a truck. The captain could pull a handle on the instrument panel to release pressurized air into the hydraulic brake system in the event of hydraulic pressure loss, as explained in this AAIB bulletin. The fuselage air brakes were removed from the first prototype, no DC-8 entered service with them. Although the wing spoilers are not used as speed brakes in flight, reverse thrust is allowed on the inboard engines during descent.

Ah, that makes sense (although, in that case, the AAR should've referred to them as "air brakes" [air-actuated brakes], not as "airbrakes" [aerodynamic brakes]).


text:
How much compensation does an unruly passenger have to pay to an airline if the flight has to be diverted to remove them?
If a passenger becomes unruly and the captain decides to land and remove that passenger from the flight, how much does the passenger have to pay the airline for the cost?
In addition, what are the reasons for flight crews to decide whether to divert or continue the flight?

diverting is for emergencies, a passenger that is a danger to the crew, other passengers or even himself is an emergency

The usual term is "unruly" passenger.

Note that such passenger is usually in bigger trouble than just having to pay something. They are often arrested and may face criminal charges as well.

Well noted @janhudec. In addition of criminal charges, civil charges are often overlook and forgotten. Any idea how large the sum might be? One possible cost i can think of is hotel bill for the whole plane if flight crews exceed their working hours...

@vasin1987 that no doubt is extremely regional. Civil charges might even be impossible if criminal charges are filed (IANAL, but heard such is the case in some countries).

@jwenting: Sounds hard to believe. That would open up an easy way to duck out of civil damages. You might need to attach the civil suit to the criminal trial so it's tried as a single act, but that's just paperwork.

Unruly passengers can face the following:

Monetary fines
Prison sentences
A ban from that airline

Excerpt from Wikipedia:

In the United States, passengers who disrupt the duties of a flight
  crew member can face fines up to $25,000 and sometimes lengthy prison
  sentences.

IATA has a policy for these passengers mentioned here, and more information is here.
ICAO has a detailed document on unruly passenger prevention and management. They have adopted a new protocol to amend the Tokyo Convention.

Note that these fines aren't always/often to the airline, but are often criminal fines imposed by the government for breaking the law. An airline doesn't usually have the power to fine an individual directly, they would have to file a lawsuit

That's what I was thinking @JonStory. It's a useful answer to consider the criminal penalties, but I'm wondering if there are any limits on the civil penalties. I think that would make this answer more complete. I'm guessing no such limits?

There are usually limits on civil penalties, but there aren't many limits on civil damages. So it depends whether the airline chased a fine (civil penalty) or damages/losses.


text:
Why ETOPS 207 minutes?
There are many ETOPS certification levels, such as 120 (2 hours), 180 (3 hours), or 300 (5 hours).  However, what is the reason behind ETOPS 207?  Is there anything specific at that point?  Why not make it ETOPS 210 (3.5 hours) altogether?

ETOPS 138 and 207 are 15% extensions of ETOPS 120 and 180.
The reason for that is that with the ETOPS 120 rule, most of the Atlantic could be covered when using Kangerlussuaq (Greenland), Gander (Newfoundland), Lajes Field (Azores) and Keflavik (Iceland) as alternative airports. Only some small triangles of the Atlantic were outside the 120 minutes flight time limit from these airports. To cover the whole Atlantic, the next step up in ETOPS (180) would be necessary. But since that is such a big difference, a 15% extension of 120 minutes, ETOPS 138, was approved which allows for covering all of the Atlantic with few extra requirements.
For ETOPS 180 it is similar, but then for the Pacific. Normally ETOPS 180 is sufficient to cross the Pacific, but ETOPS 207 allows the 777-200ER/-200LR/-300ER to cross the Pacific ocean even when Eareckson Air Station on the western end of the Alaskan Aleutian Islands is closed.

There is nothing specific as such. It was given mainly as an extension to the 180 min ETOPS in North Pacific Area on a case by case basis. From 14 CFR Part 121, Appendix P to Part 121 - Requirements for ETOPS and Polar Operations:

207-minute ETOPS in the North Pacific Area of Operations. (1) The FAA grants approval to conduct ETOPS with maximum diversion times up to 207 minutes in the North Pacific Area of Operations as an extension to 180-minute ETOPS authority to be used on an exception basis.
This exception may be used only on a flight-by-flight basis when an ETOPS Alternate Airport is not available within 180 minutes for reasons such as political or military concerns; volcanic activity; temporary airport conditions; and airport weather below dispatch requirements or other weather related events.

Basically, ETOPS 207 (180 min + 15 percent) is an operational extension of ETOPS 180 (before ETOPS 240 and 330 were certified) to be used in case a 180-min alternate airport was not available due to some reason.
From Boeing Aero:

Note that 207-minute ETOPS is not subject to the new ETOPS requirements for "beyond-180-minute flight operations.” Flown since 2000, this authority arose as a 15 percent operational extension, for limited use on an exception basis, to 180-minute ETOPS authority. it is thus considered an extension of and subject to the requirements for the traditional 180-minute “twinjet ETOPS” diversion authority.

Note that there is a similar ETOPS rule over North Atlantic routes - the 138 min ETOPS, which is nothing but ETOPS 120, plus an additional 15% in case alternate airports are not available (if I'm not wrong, for the Atlantic, the alternate airports are Keflavik and Lajes).


text:
Is there a special procedure for mid-air refueling in the rain?
After reading about mid-air refueling and watching The Perfect Storm, I have a question: is there a special procedure to observe when performing an in-flight refueling in the rain?
Side note: I heard the tanker pilot says something like "we have a wet hose". Is this phrase common?

"wet hose" probably means that there is fuel in the hose (which people should be aware of) rather than being wet from rain.

There is a procedure: don't.

It always helps to add to questions like this, why you think that there might be something different.  Rain is rain, but not all rain is equal.

@GdD I realise that was a comment rather than an answer but, still, more detail would be helpful.

@simon maybe increase risk of gusting in the rain require extra care? Maybe a condition not known by non pilot like me :)

Air refueling in many cases is done at an altitude above any weather, so this is only rarely a factor, if ever

@SSumner the case I saw in the movie was under weather. I agree they prefer to do it above weather but when they have no choice and have to do it below? Anything special for that?

Refuuuuuueling in the raaaaain...

If you're VMC and simply have rain falling, the procedure doesn't change, and it isn't a big deal (talking fixed wing refueling with a boom here; other communities' procedures/capabilities may be different).  The receiver's windshield will have some amount of rain hitting it, but if it isn't particularly heavy rain then he should still be able to do the AR.
If the visibility is really bad because of the rain, or if you're getting into IMC, then that's the situation you have, irrespective of rain falling or not, and obviously you'd rather have better conditions for doing the refueling.  Going IMC momentarily while you're hooked up may not be a big deal, as you can still see the tanker, but if you lose sight then the first priority is to get positive separation between the two aircraft.

The procedure is no different to if it's not raining unless the rain is heavy enough to impair the visibilty from the receiver aircrafts cockpit.
Air to Air refuelling is normally planned to be carried out above cloud but there may be operational reasons to do it elsewhere
The terms wet and dry refer to whether fuel is flowing or not.  A wet contact is one where the receiver receives fuel from the tanker.  A dry contact is one where contact is made but no fuel is transferred, this is done for training as the process of making and breaking contact needs to be practiced


text:
What type of fuel is used in helicopters?
What type of fuel is used in helicopters?
Do they use Aviation Turbine Fuel (jet fuel), which is used widely in commercial airlines?
Also, where would be the fuel tank located in a helicopter?

Commercial airlines typically use Jet-A/1

It depends on the engine. A piston engine will use avgas (aviation gasoline) or diesel (or Jet-A for availability reasons) depending on whether the engine is a diesel engine (which can burn jet fuel), while a turbine engine will use jet fuel. Essentially whatever the engine is rated to burn.
The fuel tank is somewhere in the fuselage near the center of the rotor. This to avoid the center of gravity shifting with the fuel level.

A Diesel helicopter? I've never heard of such a thing.

@Gabe: Looks like some work has been done in that area, though: http://www.wired.com/2010/02/europeans-love-their-diesel-helicopters-are-next/

How long before we have helicopters parked on off-ramps overnight... (couldn't resist. :)

wired is hardly a credible source for helicopters! there aren't any certificated diesel helis

Besides, aviation diesels burn Jet-A anyway.

That's the main reason for having a diesel in an aircraft.  Diesel fuel doesn't work that well as aviation fuel.  But diesels can run on Jet A, which is widely available and cheaper than the older, 100-octane, leaded avgas.  The latter is getting harder to find and more expensive with each pass year.  Considering the fact that 2014 was SUPPOSED TO be the year they phased out leaded aviation fuel altogether, that's not surprising.


text:
How do hot air balloon pilots avoid collisions?
There are many places in the world where hundreds of hot air balloons are flown in close proximity to each other. How do the pilots steer them away from each other to avoid mid-air collisions?
Wind blows all of them in one direction and I'm sure there are plenty of calculations done by these expert pilots prior to taking off, but I am also sure that not all the balloons will react to wind at exactly the same rate.
What happens when they are getting too close for comfort? Do they have horizontal movement control?

Credits: NewYorkUpstate.com
Ironically this image already has one collision in it on ground.

"I am also sure that not all the balloons will react to wind at exactly the same rate." Wind is just air moving, and the balloons just move with the air. If you imagine all the air to be moving North at 2 miles per hour, all the balloons will move North at 2 miles per hour.

Surely if they do collide, it can't be that serious, right?

Darwinism?  :-)   .

I'm curious where you see the collision on the ground.

@FreeMan, spot the black balloon at the left-hand side that is practically in the frame. Then to the right of that one, there are three balloons apparently in a line. The two leftmost of those most certainly do seem to be in contact as they both erect themselves, and possibly the middle one contacts the third as well.

@David regardless of their surface area and mass and also some shape differences?

@HankyPanky Yes, regardless. The balloon is attached to the air and will move as the air moves.

@DavidSchwartz I'm afraid that's very simplified. Physics won't agree to it per see. It is true that the balloon will eventually reach the speed of wind and move as fast as the wind but the time it takes to reach there is going to be different for balloons of different mass and different drag coefficient and different surface areas

@DavidSchwartz In addition, where I'm living wind rarely blows at constant velocity over scales relevant to balloon collision, especially when there's balloon-like obstacles around that create strange perturbations.

Also if it is that simple, why I don't start moving at speed of wind when jumping?

I grew up in a family hot air ballooning business, and while I haven't been involved in a few years, I can answer your question in two words: They don't!
Taking Off
As far as collisions go, the other answers have mostly addressed this: hot air balloons only control vertical movement, so with all the balloons experiencing the same wind, they'll move at approximately the same speed.
When taking off, we always angled the balloon so that the wind would blow from basket to envelope. In a large group, such as at festivals, we would maintain the same practice as much as possible, and have a line of balloons perpendicular to the direction of the wind. Often, this would restrict the space enough that inflating balloons would roll slightly into adjacent ones, but with a competent/strong person on the crown-line (the rope attached to the top of the balloon, used to maintain stability in the event of crosswind), the small gusts which caused localized rolling could be overcome.
In large groups, when a balloon is ready to lift off, the pilot will typically ensure nobody is going to be directly overhead, then apply more heat to lift out quickly. This prevents the basket from catching any of the envelopes of the balloons still on the ground. As mentioned, once in the air, collisions are not really a problem.
EDIT: I believe you can see two balloons on the ground but vertical which are pressed up against each other in the picture you've provided. Try looking near the middle, just to the top left. You can see that the envelopes are harmlessly pushing away from each other while the baskets are still quite far apart.
In The Air
Balloons are huge and move slowly, particularly slowly relative to each other. If two balloons are exactly level and one directly blocks the wind to the other when a gust comes, it's possible for one to suddenly gain speed. I've never seen gusts strong enough or long-lasting enough to create a speed difference of more than a couple miles per hour, so when the balloons do touch, the envelopes gently 'kiss,' then bounce back. This can, at worst, create a slow rocking of the basket.
Now, the last situation, if a pilot is unaware of another balloon directly overhead and starts going up, it could be a problem. However, pilots typically have radios and can communicate with each other if it's necessary. The pilot in the unseen balloon above would start applying heat to try to get out of the way as well as radio some (likely unkind) choice words to the pilot below him.
Landing
This is more of an after-thought, but if a pilot wants to land and another balloon is already there, the pilot will simply stay in the air a little longer to find another landing site.
Competitions
A second after-thought, but I know about 10+ years back there were a couple competitions at festivals where pilots would receive a special ribbon device and a partner-balloon. The device would mark the longest the ribbon had to extend. Each of the pilots would attach one end to their basket, then the duo who stayed the closest to each other would win a prize.
Another type of competition also occurred in which the pairs of pilots would take off about 15-30 minutes apart and the second pilot would have to try to land as close to the landing site of the first one as possible. Sometimes, this meant landing at roughly the same time and in uncomfortable proximity. Safety is always the first concern of a good pilot, though! If a pilot wasn't sure it could be done safely, he or she wouldn't do it!
Summary
As longwinded as this is, the gist of the answer is that balloon-balloon collisions aren't very serious unless the basket is involved. Typically, they move slowly enough to make it easy to avoid unwanted collisions, despite their lack of horizontal control.

This! This is what I was about to start a bounty for. I wished to see an answer from someone that did it for a living

And welcome to the site:) a very nice first post

Thanks! I was around hot air balloons most afternoons from the age of 5 to 15, feel free to message me if you have any other questions about ballooning or if you feel my answer could be improved!

@HankyPanky After a certain amount of time, you can still post a bounty. One of the reasons given for bounties is to reward exceptional answers. I, myself, have done this once.

You can also see very clearly in the bottom right there's two balloons still on their trailers with their envelopes touching.

The perspective on that photo makes it hard to tell, but it looks like many of the balloons are still on the ground.  So it's not quite as chaotic as it appears.
There is no direct horizontal control.  All they can do is ascend or descend to catch the preferred wind.

How do they steer those away from each other to avoid mid air
  collisions?

For the most part, they don't.  The same wind is blowing them all, so if you launch into a clear space, it's pretty difficult for some other balloon at the same altitude to join you.

I am also sure that not all the balloons will react to wind at exactly the same rate.

Exactly? No, but pretty close.  It also means that if two craft do come together, it will be at a pretty low closing speed.  When they are at the same altitude, the envelopes might bump, but that shouldn't hurt anything.  The danger that they want to avoid is ascending or descending into another balloon.

If the envelopes bump, how much hot air is displaced, to be replaced by cold (dense) air after the collision? How much altitude is lost due to this, and at what rate? Might this happen to one of the balloons involved more than the other, such that one balloon will be above the other soon after the collision?

@dotancohen Yes the smallest one would have the greatest displacement, and since it had less hot air volume to start with, it would lose the most hot air in your scenario, and would sink faster.

@dotancohen, It would depend on the relative speed, but it's not like squeezing a bottle, think more like squeezing a pillow.  There can be a bit of deformation before there's a significant volume reduction.  Here's a small one that has little effect.  https://www.youtube.com/watch?v=z0BSR0r9Mng

The balloons just float, there is no thrust so no wake etc. If there is a constant wind, all balloons have exactly the same speed. Only local phenomena can create differences in horizontal relative distances, such as:

Wind gusts and wind direction changes
Wind shear
Local updrafts

A local difference in wind speed will accelerate first one, then the second balloon to the new wind speed due to differences in inertia from mass. But only momentarily.
Wind velocity does change with altitude, and the best chance of a horizontal speed difference between two balloons is when there is a vertical speed difference. This was the cause of the deadliest balloon mid-air collision, the one near Alice Springs in 1989, killing 13 people. It was the result of one balloon climbing into another balloon: a difference in vertical speed initiated by the lower balloon. The company practise was for the upper balloon to give way to the rising lower balloon, which was not followed in this case.
But if I may make my choice of colliding vehicles, I would definitely choose to be in one of two horizontally colliding balloons in full flight - slowly wafting against each other.

Seeing straight up would be pretty hard in most balloons - How would you defend against this?  clear panel in the top of the balloon?  Small cameras?  Or just keeping your eyes open and remembering?

@Criggie yeah, the company that operated the Alice Spring balloons had a rule that the top balloon must give right of way to the lower balloon, according to the wiki.

How do they give way? Just rise as fast or faster than the lower balloon all while trying to get their attention?

@Brad good point, yes i guess the only way is up.

You can always go up.  And eventually, when you go up too far, you hit the jetstream and then you get out of the way really quickly ;-)

@CortAmmon, of course everyone has expired from hypoxia and/or hypothermia by that point, but let's not quibble over details. ;)

@Criggie the top of a hot air balloon is not (always?) closed, right. So you can look up I think. https://upload.wikimedia.org/wikipedia/commons/5/51/Top_Venting_Of_A_Hot_Air_Balloon.jpg though I guess the holes are not always open. Not sure how it works but maybe you can use it to look up before starting your ascend.

I suspect some balloons are more aerodynamic than others - the custom shaped ones might present more or less surface area to the prevailing wind than a conventional balloon-shaped balloon.

"Only wind gusts and wind direction changes can create differences" and wind shear. Normally, the higher, the faster the wind. https://en.wikipedia.org/wiki/Log_wind_profile

@Criggie yes, but in a uniform flow field all balloons accelerate/decelerate to the wind velocity, regardless of shape or surface. Only when there is a gradient would that make any difference, for instance if the wind @ 10 m higher is stronger, and one balloon is 5 m tall and another 8 m tall. The difference in balloon height causes a difference in horizontal speed then. Or local updrafts, gusts, jet blasts etc.

Please don’t mind unaccept on this answer. It’s a great answer and I have upvoted it but I’ve decided to reward a new user’s equally good answer

:) absolutely no worries mate, and it's good to have an experienced ballooner on the site. I've up-voted it as well.

@RoyT. If you vent the balloon like that, you're definitely not going to be going up any time soon! :P

the lower balloon has the right of way at least in the US.  Likewise balloons have the right of way over all other aircraft.  Balloons can climb at an alarming rate compared to fixed wing, fixed wing pilots often make bad assumptions related to balloons because they have a relatively limited climb rate.  you can decend quickly too but have to recover of course.  so you dont have to see above you, but during an event like this the rules often include a max rate of climb rate of say 200fpm when in congested space, you can hear the balloons above you burning so you know they are there

you can often see their shadow to on the ground over your shadow to get a rough idea of where they are

authors: Vladimir F Героям слава


text:
How far ahead of the wing should a propeller be located for a UAV?
This question is asked in context of a fixed wing, Unmanned Aerial Vehicle (UAV), with a wing span of about 5m (16ft).
How far ahead of the wing leading edge should the propeller be located?
The propwash should have minimal effect on stability and should not adversely affect the airflow over the wing.
Can propwash be useful?

Your question is a bit vague. Do you want the props as close to the wing as aerodynamically possible, or as far from the wing as possible?

Are you designing a twin? Because with single engine you don't have much choice: you must place the prop at the nose (or tail; pusher is a choice you do have) and the wing according to the centre of gravity.

Why does it have to be ahead of the leading edge - I can think of a few where it is to the rear.

Sorry for not being clear.

I am only looking for a tractor configuration, for now. It is a single prop not twin. So, I want to know what should be the distance between the propeller and wing leading edge so as to minimise the disadvantages of propwash. Is there any rule of thumb or empirical relation or any relevant material on this? Re~ 5e5.

what would be the changes if a twin prop is used? Span wise location will also come into the picture but i dont think there will be any cancellation of propwash taking place. All the views will help. Thanks.

@Dcode any update on how this turned out? what you discovered regarding optimal prop distance and flight characteristics, noise, etc

https://www.reddit.com/r/aerospace/comments/4dd98q/distance_between_propeller_and_leading_edge_of/ also based on measuring images of random prop planes, the distance between the prop and the leading edge of the wing seems to be between around 8% and 15% of the total wingspan. 8% for larger planes